1. Field of the Invention
The present invention relates generally to recording apparatus for an optical recording medium. More particularly, the present invention relates to a recording apparatus for recording audio data or general data on an optical recording medium.
2. Background of the Invention
Erasable discs on which the user can write or record different musical data or audio data repeatedly are known. One such known erasable disc is a magneto-optical disc. To record data on the magneto-optical disc, a magnetic head is used to apply a vertical magnetic field from one side to a recording location of the disc while heating the same recording location above the Curie temperature by irradiating the magneto-optical disc from the opposite side with a light beam emitted from an optical head, for example, at a higher output level than in the reproducing or playback mode.
There are two systems currently used for recording information signals onto the magneto-optical disc. In one recording system, recording data based on the information signals are supplied to a drive circuit for a light source of the optical head so as to modulate a light beam according to the recording data, and an external magnetic field of the magnetic head is applied with a constant intensity and in one direction. In the other recording system, the recording data are supplied to a drive circuit of the magnetic head for enabling the magnetic head to output an external magnetic field such that the direction of N and S poles of the external magnetic field is reversed according to the recording data, and a light beam is continuously emitted from the optical head at an output level which is required for the recording operation.
Hereinafter will be described a recording apparatus for magneto-optical discs, in which the latter-mentioned recording system is employed.
In the recording apparatus, a signal, such as an audio signal, to be recorded on a magneto-optical disc is subjected to a predetermined modulation process and then supplied as recording data to a magnetic head. The magnetic head generates a vertical magnetic field based on the recording data and applies the vertical magnetic field to the magneto-optical disc. At the same time, an optical head irradiates the magneto-optical disc with a light beam to heat a recording layer of the magneto-optical disc at a temperature above the Curie temperature thereof. Since the magneto-optical disc and the light beam move relatively to one another, the temperature of the recording layer drops below the Curie temperature during which time the recording layer is magnetized in accordance with the direction of the vertical magnetic field applied from the magnetic head. As a result of this magnetization, the audio signal is recorded on the magneto-optical disc.
In the above-described recording operation, a light beam of a reproducing or playback level (i.e., an output level required for the reproducing or playback operation) is emitted from the optical head onto the magneto-optical disc to read address information previously recorded on the magneto-optical disc, and the optical head is moved or transferred to a desired position on the magneto-optical disc. With this accessing operation, the light beam emitted from the optical head is caused to arrive at a desired recording start point or position whereupon the output level of the light beam emitted from the optical head is changed to a recording level which is an output level required for the recording operation. At the same time, the vertical magnetic field based on the recording data is applied from the magnetic head to the magneto-optical disc in the manner described above.
However, when the output level of the light beam is changed to the recording level upon arrival of the optical head and, to be exact, the light beam at the recording start position, if a defect, such as a flaw, is present at the recording start position, the recording operation will be achieved inaccurately. A reason for such inaccurate recording will be given below in greater detail.
In general, the optical disc recording apparatus reads address information from the light beam reflecting from the disc and also extracts a focus servo signal and a tracking servo signal from the reflected light beam during an accessing operation regardless of whether the accessing operation is done for the recording operation or the reproducing or playback operation. For these signals, an automatic gain control (AGC) circuit is provided in order to obtain pieces of extracted information, such as focusing information and tracking information, in terms of signals having a constant amplitude level. One such AGC circuit is shown in FIG. 1.
An optical head or an optical pickup 31 shown in FIG. 1 includes a photodetector (not shown) for detecting the light beam reflected by the magneto-optical disc. The output from the photodetector is subjected to an operation process through which an RF signal S.sub.ABCD, a tracking error signal TE, and a focusing error signal FE are outputted. These signals S.sub.ABCD, TE and FE are supplied to a sum signal amplifier 32, a tracking error amplifier 33 and a focusing error amplifier, respectively.
The RF signal S.sub.ABCD amplified by the sum signal amplifier 32 is supplied to an address decoder unit, a data decoder unit (neither shown), and a time constant circuit 35 composed of first and second resistors R.sub.1, R.sub.2 and a capacitor C. The output signal of the time constant circuit 35 is fed back, as an AGC control signal C.sub.AGC, to the sum signal amplifier 32, the tracking error amplifier 33 and the focusing error amplifier 34 for controlling the gain of the respective amplifiers 32, 33, 34. As the level of the AGC control signal C.sub.AGC increases, the gain of each of the amplifiers 32, 33, 34 decreases. Conversely, the gain of each amplifier 32, 33, 34 increases with a decrease in the level of the AGC control signal C.sub.AGC. By virtue of the feedback control, the output from each of the amplifiers 32, 33, 34 is controlled at a constant level. Thus, amplitude levels of the focusing error signal FE and the tracking error signal TE are controlled constantly.
The time constant circuit 35 includes a switch 36 which is constructed to select a first terminal T.sub.1 or a second terminal T.sub.2 depending on a switching control signal C.sub.SW supplied from a controller (not shown) such as a microcomputer. When the terminal T.sub.1 is selected, the time constant of the time constant circuit 5 is set to a relatively large value due to the combination of the resistors R.sub.1, R.sub.2 and the capacitor C. In contrast, when the terminal T.sub.2 is selected, the time constant of the time constant circuit 5 is set to a relatively small value due to the combination of the resistor R.sub.1 and the capacitor C.
A consideration will be given to a moment at which the output level of the light beam is increased to the recording level upon arrival of the light beam from the optical head at the recording start position. In this instance, since the quantity of the reflected light beam increases with an increase in the output level of the light beam, the amplitude of the tracking error signal TE and the amplitude of the focusing error signal FE are increased. To accomplish a normal servo operation, the amplitude of the error signals TE, FE must return to the initial level as quickly as possible. To this end, when the output level of the light beam is to be increased, the terminal T.sub.2 of the switch 36 is selected to reduce the time constant of the AGC control signal C.sub.AGC, thereby achieving the AGC control rapidly.
In contrast, during continued recording operation continues or in the reproducing or playback mode, the terminal T.sub.1 of the switch 36 is selected to increase the time constant of the AGC control signal C.sub.AGC in order to insure a continued recording or reproducing operation without undue fluctuation of the signal level even in the presence of a slight damage on the magneto-optical disc.
Now, we assume that a recording track shown in FIG. 2(a) has a defect Def, such as a flaw, existing at a recording start point or position where the laser power is changed to a high level, as shown in FIG. 2(b). In this instance, due to the influence of the defect Def, the detected quantity of the light beam reflecting from the magneto-optical disc is reduced. However, when the laser power is increased, the time constant of the AGC control signal C.sub.AGC is set to have a small value under the control of the switching control signal C.sub.SW shown in FIG. 2(c). Accordingly, the AGC control signal C.sub.AGC instantaneously follows up an increase in the laser power, thereby increasing the gain of the amplifiers 32, 33, 34 abruptly. Since the time constant is returned by the switching control signal C.sub.SW to the ordinary level immediately after the completion of shifting-up of the laser power, it takes time until the gains of the amplifiers 32, 33, 34 drops to a proper level. The gain of the thus AGC controlled amplifiers 32, 33, 34 is as shown in FIG. 2(d).
Accordingly, the amplitude level of the tracking error signal TE, for example, becomes unstable, as shown in FIG. 2(e), so that the servo operation is performed unstably. The same may be said of the focusing error signal FE. Under these circumstances, an operation failure such as a defocusing or a track jump is likely to occur, disabling the recording apparatus from continuing the normal recording operation.